This application is based on Japanese Patent Application No. 11-69908 filed Mar. 16, 1999, and No. 2000-51779 filed Feb. 28, 2000, the contents of which are cited herein by reference.
The present invention relates to a method and apparatus for manufacturing a control rod for a nuclear reactor, the parts of the control rod are adhered to one another by laser welding or TIG welding.
FIG. 1 schematically shows an example of the internal structure of a reactor pressure vessel (RPV) of a boiling water reactor (BWR).
Referring to FIG. 1, water as a coolant is heated and boiled, while flowing upward from a bottom portion of a reactor core through a gap between fuel rods 1 containing fuel. Power of the reactor varies in accordance with withdrawal or insertion of a control rod 2. The reactivity is increased when the control rod 2 is withdrawn. The reactivity is decreased when the control rod 2 is inserted.
An example of the structure of the control rod 2 will be described with reference to a schematic perspective view of FIG. 2A. As shown in FIG. 2A, the control rod 2 mainly comprises blades 3, a handle 4 attached to the upper ends of the blades 3 and lower-blades 5 attached to the lower ends of the blades 3. More specifically, the blade 3 comprises neutron absorbers 6, a sheath 7 housing them and insertion pieces 8 inserted for the purpose of securing a portion to allow water to flow between the neutron absorbers 6 in the sheath 7.
Four of the blades 3 make a set, and the adhesion ends of the respective blades are connected to a tie rod so as to form a cross shape as a whole.
Conventionally, the control rod 2 of the reactor is manufactured by the following method. FIGS. 3A and 3B show examples of processes for manufacturing the control rod 2.
First, a process for manufacturing the control rod 2 using boron carbide (B4C) as the neutron absorbers 6 will be described with reference to the flowchart of FIG. 3A.
In this case, first, the periphery of the sheath 7 is cut (S1), and weep holes are formed by cutting (S2). Thereafter, the sheath 7 is bent like a C-shape (S3). After the ends of the C-shaped sheath 7 are aligned, a tube containing boron carbide as the neutron absorbers 6 is inserted in the sheath 7 (S4). Thus, the blade 3 is assembled.
The blades 3 are fixed to the tie rod 9, with the result that the control rod 2 is assembled (S5). Thereafter, welding of the tie rod 9 (S6) and welding of the handles 4 and the lower-blades 5 (S7) are carried out. Finally, the control rod 2 is smoothed by finishing with a wire brush or the like (S8).
No insertion piece 8 is employed in the control rod 2 which uses B4C as the neutron absorbers 6.
In the above case, B4C is used as the neutron absorbers 6. Next, the control rod 2 using hafnium (Hf) as the neutron absorbers 6 will be described.
In this case, as shown in FIG. 3B, the periphery of the sheath 7 is cut first (S11), and weep holes are formed by cutting (S12). Thereafter, the sheath 7 is bent like a C-shape (S13). After the ends of the C-shaped sheath 7 are aligned, piece-inserting holes through which the insertion pieces 8 are inserted inside the C shape is formed by cutting (S14). Thereafter, an Hf plate as the neutron absorbers 6, and the insertion pieces 8 are inserted in the sheath 7, thereby assembling the blade 3 (S15). The sheath 7 and the insertion pieces 8 are welded together (S16) and finishing with a wire brush is carried out (S17), thus producing the blade 3.
Further, the tie rod 9, the handles 4 and the lower-blades 5 are combined with the blades 3 produced by the above method, with the result that the control rod 2 is assembled (S18). Thereafter, welding of the handles 4 and the lower-blades 5 (S20) are carried out. Finally, the control rod 2 is finished with a wire brush (S21).
In the case where the control rod 2 of the reactor is manufactured as described above, first, the sheath 7 is cut and bent, and thereafter the blade 3 is assembled. Then, the frames of the tie rod 9, the handles 4 and the lower-blades 5 are assembled. Further, welding of the blades 3, the tie rod 9, the handles 4 and the lower-blades 5 and welding of the sheath 7 and the insertion pieces 8 are carried out.
Conventionally, welding of these parts is performed by manual TIG welding. The TIG welding is a method in which arc is generated between a nonconsumable tungsten electrode and a base material in an inert gas atmosphere for the purpose of melting.
According to the manual TIG welding, since heat input is great, deformation after the process is great, and in particular, welding deformation is considerable. Therefore, welding is frequently performed while the deformation is corrected. In addition, since the process speed of welding is low, the productivity is low.
An object of the present invention is to provide a method and apparatus for manufacturing a control rod for a nuclear reactor, by reducing heat input to suppress deformation after the process and increasing welding speed to improve the productivity.
The above object is obtained by the invention as described below.
(1) In a method for manufacturing a control rod for a nuclear reactor comprising blades including neutron absorbers, insertion pieces and a sheath, weep holes and piece-inserting holes are formed in the sheath by cut working and the periphery is cut. Thereafter, the neutron absorbers and the insertion pieces are inserted in a recess portion formed by bending the sheath. The piece-inserting holes and the insertion pieces are welded together by a laser beam, thereby forming an integral blade. The blades and handles are welded by a laser beam, and subsequently, the blades and lower-blades are welded by a laser beam. Then, the welded portions are surface-finished by a wire brush or the like.
(2) In a method for manufacturing a control rod for a nuclear reactor comprising blades including neutron absorbers and a sheath, weep holes are formed in the sheath by cut working. After the periphery is cut, the neutron absorbers are inserted in a recess portion formed by bending the sheath, thereby forming an integral blade. The blades and handles are welded by a laser beam, and subsequently, the blades and lower-blades are welded by a laser beam. Then, the welded portions are surface-finished by a wire brush or the like.
With the method for manufacturing a control rod described above, the heat input in the welding time is kept low and the deformation after the process is suppressed. Moreover, since the process speed of welding is increased, the productivity can be improved.
(3) In the welding time, to reliably melt a groove line and secure the penetration bead on the rear surface of the sheath, a YAG laser beam or a CO2 laser beam is irradiated on a position located immediately above a groove position or a position deviated in parallel from the groove position within a range of 2 mm. The laser beam may be irradiated several times so as to lap over one another, shifted within a range of 0.1 to 2.0 mm from a first pass or a first pass target position. Alternatively, the laser beam is irradiated while traveling forward with circular motion in a diameter within 2 mm. To produce circular motion of the laser beam, a prism is inserted before a condenser lens, and while the prism is rotating on an axis of the laser beam, the laser beam is irradiated on the rotating prism. As a result, the laser beam is incident on the condenser lens, while rotating.
With the above method, in any case, the groove line is reliably melted and the penetration bead on the rear surface is secured.
(4) As another method, in the step for welding the blade to the tie rod, the blade to the handle or the blade to the lower-blade, two slits are provided in the tie rod, the handle or the lower-blade, and projections of the blade is inserted in the slits. The laser beam is irradiated on a boundary between the blade and the tie rod, a boundary between the blade and the handle or a boundary between the blade and the lower-blade.
With the above method, in any case, the groove line is reliably melted and the penetration bead on the rear surface is secured. In addition, the groove line is not deviated due to the work accuracy of the blade. Therefore, the laser beam is reliably irradiated on the groove line.
(5) A CCD camera is mounted on the work head, and the CCD camera in its entirety is moved to the groove position as designed. An image near the welding portion is picked up by the CCD camera. The image processing apparatus detects the actual groove position from the image, i.e., the coordinates of the center of the insertion piece from the groove position in the case of welding the piece-inserting hole of the sheath and the insertion piece. In the case of welding the blades and the tie rod, welding of the blades and the handles or welding of the blades and the lower-blades, the work start and end points of the welding are detected from the groove position. The arithmetic processing apparatus compares the actual groove position with the groove position as designed, and calculates the deviation of the irradiation position of the laser beam. After the work head is moved by a distance corresponding to the amount of the deviation of the irradiation position of the laser beam plus the amount of the offset of the work head and the CCD camera, the laser beam is irradiated.
With the above apparatus for manufacturing a control rod, even if there is a difference between the groove position as designed and the actual groove position, the groove line is melted reliably, the penetration bead on the rear surface of the sheath is secured, and the welding process can be automated.
In addition, since the heat input in the welding time is kept low, the deformation after the process is suppressed. Moreover, since the process speed of welding is increased, the productivity can be improved.
(6) In an apparatus for manufacturing a control rod of a nuclear reactor, the control rod comprising blades including neutron absorbers, an insertion pieces and a sheath or blades including neutron absorbers and a sheath, a tie rod for fixing the blade, a handle and a lower-blade fixed to the tie rod and the blade, there is provided a work table, a laser oscillator, a work head, a CCD camera, an image processing apparatus, an arithmetic processing apparatus, a servo motor and control means. The above apparatus comprises at least two work he ads and two work tables and a movable mirror between the laser oscillator and the work heads.
The work table is used to set objects to be welded constituting the control rod and carry out welding. The laser oscillator outputs a YAG laser beam or CO2 laser beam. The work head has an optical system for transmitting the laser beam oscillated by the laser oscillator and converging it to the objects to be welded by a condenser lens. The CCD camera, mounted on the work head, picks up an image of a portion near a groove of the objects to be welded. The image processing apparatus detects a groove position from the image picked up by the CCD camera. The arithmetic processing apparatus compares the groove position obtained by the image processing apparatus with a groove position as designed, and calculates deviation therebetween. The servo motor moves the work head or the work table by a distance corresponding to an amount of the deviation of the groove position calculated by the arithmetic processing means plus an amount of the offset of the work head and the CCD camera. The control means drives and controls the servo motor.
With the above apparatus for manufacturing a control rod, the welding process can be automated. Further, since the groove position is adjusted, the groove line is reliably melted, the penetration bead on the rear surface of the sheath is secured and the welding process can be automated. In addition, the heat input is kept low and the deformation after the process is suppressed. Moreover, since the process speed of welding is increased, the productivity can be improved.
(7) In a method for manufacturing a control rod for a nuclear reactor, weep holes and piece-inserting holes are formed in the sheath by cut working and the periphery is cut. Thereafter, the neutron absorbers and the insertion pieces are inserted in a recess portion formed by bending the sheath. The piece-inserting holes and the insertion pieces are welded together by TIG welding, thereby forming an integral blade. The blades and handles are welded by TIG welding, and subsequently the blades and lower-blades are welded by TIG welding. Then, the welded portions are surface-finished by a wire brush or the like.
With the method for manufacturing a control rod described above, the heat input in the welding time is kept low and the deformation after the process is suppressed. Further, if two welding torches are used simultaneously, the productivity can be improved.
(8) In the welding time, to reliably melt a groove line and secure the penetration bead on the rear surface of the sheath, automatic TIG welding is carried out at a position located immediately above a groove or a position deviated in parallel from the groove within a range of 3 mm.
With the above method, in any case, the groove line is reliably melted and the penetration bead on the rear surface of the sheath is secured.
(9) A CCD camera is mounted on the work head, and the CCD camera in its entirety is moved to the groove position as designed. An image near the welding portion is picked up by the CCD camera. The image processing apparatus detects the actual groove position from the image, i.e., the coordinates of the center of the insertion piece from the groove position in the case of welding the piece-inserting hole of the sheath and the insertion piece. In the case of welding the blades and the tie rod, welding of the blades and the handles or welding of the blades and the lower-blades, the work start and end points of the welding are detected from the groove position. The arithmetic processing apparatus compares the actual groove position with the groove position as designed, and calculates the deviation of the welding torch. After the welding torch is moved by a distance corresponding to the amount of the deviation of the welding torch plus the amount of the offset of the welding torch and the CCD camera, TIG welding is carried out.
With the above apparatus for manufacturing a control rod, even if there is a difference between the groove position as designed and the actual groove position, the groove line is melted reliably, the penetration bead on the rear surface of the sheath is secured, and the welding process can be automated.
In addition, since the welding speed is stabilized, the heat input in the welding time is kept low, with the result that the deformation after the process is suppressed. Moreover, when two welding torches are used simultaneously, the productivity can be improved.
(10) In an apparatus for manufacturing a control rod of a nuclear reactor, the control rod comprising blades including neutron absorbers, insertion pieces and a sheath, a tie rod for fixing the blade, a handle and a lower-blade fixed to the tie rod and the blade, the blade is sandwiched between chillers made of two metal plates having openings only at welding groove portions so that a TIG welding torch can access thereto. With this structure, since heat generated by welding is absorbed, welding deformation can be considerably suppressed.
(11) In an apparatus for manufacturing a control rod of a nuclear reactor, the control rod comprising a blade including neutron absorbers, insertion pieces and a sheath, a tie rod for fixing the blade, a handle and a lower-blade fixed to the tie rod and the blade, there is provided a work table, a TIG torch, a CCD camera, an image processing apparatus, an arithmetic processing apparatus, a servo motor and control means. The above apparatus comprises at least two TIG torches and two work tables.
The work table is used to set objects to be welded constituting the control rod and carry out welding. The CCD camera, mounted on the TIG welding torch, picks up an image of a portion near a groove of the objects to be welded. The image processing apparatus detects a groove position from the image picked up by the CCD camera. The arithmetic processing apparatus compares the groove position obtained by the image processing apparatus with a groove position as designed, and calculates a deviation therebetween. The servo motor moves the work head or the work table by a distance corresponding to an amount of the deviation of the groove position calculated by the arithmetic processing means plus an amount of the offset of the work head and the CCD camera. The control means drives and controls the servo motor.
With the above apparatus for manufacturing a control rod, the welding process can be automated. Further, since the groove position is adjusted, the groove line is reliably melted, the penetration bead on the rear surface is secured and the welding process can be automated. In addition, the heat input is kept low and the deformation after the process is suppressed. Moreover, if the two TIG torches are used simultaneously, the productivity can be improved.
(12) In the process of bending the sheath, since the end face of the side of the sheath that is connected to the tie rod tends to be ragged, the ragged portion is removed by laser cutting with high-pressure nitrogen gas, which can perform a process at a higher work speed than machining with a work accuracy equivalent to that of the machining. As a result, the productivity can be considerably improved.
(13) In the process of bending the sheath, since the end face of the side of the sheath that is connected to the tie rod tends to be ragged, the ragged portion is removed by shearing, which can perform processing by machining in a short period of time. As a result, the productivity can be considerably improved.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.